In general, a loudspeaker is a device that converts an audio signal from its electric form to an audible sound wave. Noticeable distortions may be introduced into the audio signal during this conversion that result in a significant loss of sound quality. To improve the sound quality of the audio signal reproduced by a loudspeaker, loudspeaker equalization may be performed in which inherent variations with frequency in the amplitude, or sound level, of the sound reproduced by the loudspeaker for a given level of signal driving the loudspeaker are normalized. More specifically, loudspeaker equalization may be accomplished by filters applied to an audio signal which are designed to compensate for the loudspeaker response. Generally, to design a compensating filter, a known test signal is applied to the loudspeaker in an environment (e.g., a room, the interior of an automobile, etc.), and the output of the loudspeaker is picked up by a microphone. The whole system (pre-loudspeaker, loudspeaker, environment, and microphone) is then analyzed, for example, by comparing the spectrum of the test signal to the spectrum of the output of the loudspeaker. If the response of the microphone is known, then the joint effect of the loudspeaker and the environment can be isolated in principle and this information used to design the compensating filter.
In some instances, the compensating filter is designed to improve the quality of the listener experience by equalizing (flattening) the spectral response of an audio system (including loudspeaker). In addition, the compensating filter may be designed to result in an approximation of a desired system response target such as the spectral response of higher quality loudspeakers. The equalization is typically accomplished by reducing the volume in certain spectral regions to avoid clipping. However reducing the volume in certain spectral regions generally reduces the overall perceived volume. Reducing overall perceived volume allows more spectral contrast with the original loudspeaker spectrum. However, reducing the overall perceived volume decreases the signal-to-noise ratio. Therefore, in designing a compensating filter, a trade-off should be made between minimizing the volume reduction and maximizing the overall effect.